We ran into a situation where the market was already volatile, and a bad trade exacerbated the issue by causing a number of HFTs to take unexpected losses and withdraw from their markets, consuming further liquidity while driving prices down, which created more losses for the remaining market-makers, who had to close their positions, consuming further liquidity, driving prices further down; all of those also negatively affecting long-term investors.
This also seemed to me an example of the opportunism of HFT, where the HFT shaves the spread by a penny or two during calm markets, but withdraws (and exacerbates issues) during volatile and troubled markets, which seems to me the point in time at which liquidity provision is most valuable.
I'm not suggesting that HFT should be outlawed, nor that HFT firms should be forced to register, act, and be regulated as official market-makers, with the associated duties.
But I do note the benefits seem to come with costs.
Long-term, I doubt it matters. It seems inevitable that the provision of liquidity will become commoditized, and that the days of concerns about flash crashes will eventually disappear into the past along with $50 retail trades, and $0.50 bid/ask spreads.